(1) Field of the Invention
The present invention relates to a binder on dry sand for castings, a sand composition of moldings containing the sand binder and a method for preparing a sand mold for castings. More specifically, the binder of the present invention comprises a hydrolyzate prepared by hydrolyzing a butadiene/maleic anhydride copolymer, its neutralized substance or its partially neutralized substance, a modified substance prepared by modifying the hydrolyzate with a basic organic compound, or the like, and the sand mold for castings is manufactured by mixing an aqueous solution of the aforesaid binder with foundry sand, molding the resultant mixture, heating, drying and thereby curing.
(2) Description of the Prior Art
Heretofore, the manufacture of the sand mold for iron, aluminum catstings or the like has been carried out by blending an organic or an inorganic binder with foundry sand such as a slilica sand, molding and curing. The various binders which can be employed in such a way have both merits and demerits.
For example, if the used sand mold in which the binder is a water glass is discarded anywhere, it will contaminate the soil in the vicinity of the discarded mold, because the water glass which has adhered to the surfaces of the sand grains is alkaline. For the purpose of avoiding the problem of such an environmental pollution, the development in the sand binder comprising a material other than the water glass has been desired. As one strategy in reply thereto, much attention is paid to an organic binder the raw material of which is a self-curing furan resin or phenolic resin. These kinds of organic binders are advantageous, because it can be removed by rubbing the sands with one another or burning them after used, and the foundry sand can be reused. However, the operation of the rubbing or the like is troublesome, and in the time of the burning, a bad odor will give off inconveniently. Additionally, with regard to the organic binder, the curing rate of the binder and the mechanical strength of the sand mold remarkably vary with many factors such as properties of the foundry sand, water content, temperature, humidity, a kind and a concentration of an acid used as a curing agent, and in consequence its treatment is difficult. Moreover, the polymerization of the binder material acceleratedly makes progress gradually during storage, and thus the binder material has the drawback that it must be consumed within one year from the point of time when it has been manufactured.
As another organic binder, there has been suggested a copolymer of an .alpha.-olefin such as isobutylene and maleic anhydride. Such a water soluble polymer in which maleic anhydride and the like are used is considered to be rosy because of overcoming the above mentioned drawbacks, easily reproducing the foundry sand, and readily mixing with each of various kinds of polymers. However, by the method in which the binder comprising the above mentioned copolymer is employed, the sand mold having sufficiently great strength cannot be obtained. In view of the requirement of the high-quality and accurate castings at present, such a self-curing organic binder as the furan resin is unavoidably utilized for the sake of the middle-sized and large-sized sand molds for castings having a high accuracy, though it is still desired that the above mentioned drawbacks are overcome.